1. Technical Field of the Invention
The present invention relates generally to the field of communications and, more particularly, to an approach for mitigating interference into a digital subscriber line system.
2. Description of Related Art
Current telephone wiring connections comprised of copper twisted-pair were not designed to support the data rates or bandwidth required for new interactive services. However, since copper lines are widely available and developed, solutions to the high speed access problem have been focused on improving the performance of systems which operate over voice-band and transmit through the public switching telephone network (PSTN). Voice-band modems are now common, but the bit rates that can be supported by voice-band modems are limited to 56 kilobits per second (kbps) or less.
The Integrated Service Digital Network (ISDN) offers an alternative to analog telephone service. In contrast to analog telephone signals (known as plain old telephone service or POTS signals), which occupy the bandwidth only up to about 4 kHz, ISDN signals can occupy frequencies up to approximately 320 kHz. ISDN supports not only voice transmission, but also data channels with bit rates on the order of 160 kbps, which is an improvement over voice-band modems. Three types of ISDN are generally used. In 2B1Q ISDN, used in North America and elsewhere, signals reside in the primary band from 0 to 80 kHz. In 4B3T ISDN, common in Germany, signals reside in the primary band from 0 to 120 kHz. In time-compression multiplexed (TCM) ISDN, common in Japan, signals occupy the primary band from 0 to 320 kHz. Due to the techniques used to modulate data, all these ISDN systems generate energy at frequencies above their primary bands.
Asymmetric Digital Subscriber Line (ADSL) technology has been developed to increase the effective bandwidth of existing copper twisted-pair communication links, enabling expanded services to be provided without requiring the additional cost of replacing or updating telephone wiring connections, whether POTS or ISDN. Several varieties of ADSL exist. In one commonly used system, ADSL and POTS service are provided simultaneously on the same twisted-pair line; in this case the ADSL is referred to as “ADSL over POTS”. In another commonly used system, ADSL and ISDN are provided simultaneously on the same twisted-pair line; in this case, the ADSL is referred to “ADSL over ISDN”. Both ADSL over POTS and ADSL over ISDN operate similarly. During the initialization procedure, the ADSL system estimates the loop attenuation and channel noise. Based on these estimates and knowledge of the transmitter capabilities, the system computes the number of bits each subchannel (tone) can support at a desired bit error rate. Typically a noise margin is assumed in the calculation, resulting in a reduction in the number of bits that are supported on each tone (and thus the data rate of the system). The purpose of the noise margin is to allow the system to continue operating at or below the desired bit error rate if the channel capacity degrades during the connection up to the amount specified by the noise margin. Most systems assume a noise margin of 6 dB, which is generally effective to provide good system operation under the assumption that the noise on the line does not increase significantly during the connection.
Bit swapping may be used after a connection has been established to maintain good performance by equalizing the bit error probabilities of the tones. If the signal-to-noise ratio (SNR) of a tone degrades, one or more bits on that tone are moved to other, higher-quality tones. Bit swapping is generally effective at mitigating the effects of slow changes in the channel and noise.
Although the use of a noise margin and bit swapping are effective strategies to maintain good system performance when the channel is degraded by impairments that are, to an extent, expected in the normal course of operation, there are certain non-constant noise sources that degrade the channel rapidly enough or severely enough that the connection is abandoned, and a re-train occurs. Two examples of severe, non-constant noise sources are residual same-pair ISDN and near-end crosstalk (NEXT) from TCM ISDN.
Interference from Same-Pair ISDN
To provide ADSL on the same line as either POTS or ISDN, systems must be designed such that the frequency bands used by the two systems sharing the line do not overlap. The separation of the two signals (ADSL and either POTS or ISDN) is achieved using a splitter. The splitter has two components: a highpass filter, which passes ADSL signals but attenuates the lower-frequency signal (for example, POTS or ISDN); and a lowpass filter, which passes the lower-frequency signal and attenuates ADSL signals. Ideally, the splitter isolates ADSL from the lower-frequency signal to such an extent that interference from ADSL to the lower-frequency system and vice versa is negligible.
In practice, splitter filters are subject to common design trade-offs, such as cost vs. complexity, performance vs. cost, etc. Consequently, splitter filters may not exhibit the ideal behavior, and the level of residual ISDN at the ADSL receiver may not be negligible. Thus, ADSL can be degraded by ISDN signals that are insufficiently attenuated by the splitter lowpass filter.
Currently, most ADSL over ISDN systems transmit upstream in the band from 138 kHz to some frequency less than or equal to 276 kHz. This band corresponds to tones 32 through 63, using the standard method of indexing the ADSL tones. However, the upstream ADSL over ISDN power spectral density (PSD) masks allow transmissions as low as 120 kHz. Ideally, after the splitter, the levels of ISDN signals entering the ADSL receiver on the same line should be low enough that they do not impact the performance of the ADSL system. However, for tones around tone 32, signal corruption from interference from same-pair ISDN signals can occur due to the non-ideal nature of the splitter. If ISDN is not on when an ADSL modem initializes, the impact on ADSL when ISDN does turn on can result in a failure of the ADSL, which is forced to retrain due to an excess of bit errors and/or a negative noise margin.
Interference due to NEXT from TCM ISDN
Another commonly used ISDN system, as used in Japan, is the Time-Compression Multiplexing (TCM) ISDN system in which downstream and upstream transmissions are time-division duplexed. A primary channel bandwidth of 320 kHz is used in both the upstream and downstream directions but during different, deterministic time intervals. DSL systems, such as ADSL over POTS and ADSL over ISDN, can be deployed in the same binder as TCM ISDN but on other lines. In this case, the DSL systems operate independently of the TCM ISDN systems, and the frequency bands used by TCM ISDN and ADSL may overlap. In this configuration, if an ADSL system initializes while a potential disturbing ISDN line is inactive, and then the ISDN line becomes active after the ADSL connection has been established, the ADSL system can fail due to the high-level near-end crosstalk (NEXT) caused by TCM ISDN. As a result, the ADSL system will re-train. Such system instability is highly undesirable.